32. (Pg. 144) How must the piston be situated in the pump? 33. (Pg. 144) What other kind of pump is described? 34. (Pg. 145) How is the forcing pump constructed, as shown in plate 14, fig. 5? 35. (Pg. 145) Describe the construction and operation of the air pump, (fig. 2, plate 1.) CONVERSATION XIII. ON WIND AND SOUND. OF WIND IN GENERAL. OF THE TRADE-WIND. OF THE PERIODICAL TRADE-WINDS. OF THE AERIAL TIDES. OF SOUNDS IN GENERAL. OF SONOROUS BODIES. OF MUSICAL SOUNDS. OF CONCORD OR HARMONY, AND MELODY. MRS. B. Well, Caroline, have you ascertained what kind of pump you have in your garden? _Caroline._ I think it must be merely a lifting pump, because no more force is required to raise the handle than is necessary to lift its weight; and as in a forcing pump, by raising the handle, you force the water into the smaller pipe, the resistance the water offers, must require an exertion of strength, to overcome it. _Mrs. B._ I make no doubt you are right; for lifting pumps, being simple in their construction, are by far the most common. I have promised to-day to give you some account of the nature of wind. Wind is nothing more than the motion of a stream, or current of air, generally produced by a partial change of temperature in the atmosphere; for when any one part is more heated than the rest, that part is rarefied, the air in consequence rises, and the equilibrium is destroyed. When this happens, there necessarily follows a motion of the surrounding air towards that part, in order to restore it; this spot, therefore, receives winds from every quarter. Those who live to the north of it, experience a north wind; those to the south, a south wind:--do you comprehend this? _Caroline._ Perfectly. But what sort of weather must those people have, who live on the spot, where these winds meet and interfere? _Mrs. B._ They have most commonly turbulent and boisterous weather, whirlwinds, hurricanes, rain, lightning, thunder, &c. This stormy weather occurs most frequently in the torrid zone, where the heat is greatest: the air being more rarefied there, than in any other part of the globe, is lighter, and consequently, ascends; whilst the air from the north and south, is continually flowing in, to restore the equilibrium. _Caroline._ This motion of the air, would produce a regular and constant north wind, to the inhabitants of the northern hemisphere; and a south wind, to those of the southern hemisphere, and continual storms at the equator, where these two adverse winds would meet. _Mrs. B._ These winds do not meet, for they each change their direction before they reach the equator. The sun, in moving over the equatorial regions from east to west, rarefies the air as it passes, and causes the denser eastern air to flow westwards, in order to restore the equilibrium, thus producing a regular east wind, about the equator.